1. Field of the Invention
This invention is in the field of printed circuits and relates generally to novel compositions useful for preflux coatings for copper to protect the solderability of the copper during heating cycles.
2. Description of the Prior Art
Printed wiring boards having copper surfaces are subjected to one or more heating cycles when being assembled. Modern circuits utilize surface mount assembly technology wherein the components are soldered directly to the pads on the surface of the printed wiring board. This method employs soldering on both sides of the board and may be commingled with conventional "through-hole" assembly where the component leads are inserted in holes provided in the board. The inserted leads are subsequently soldered.
The assembly sequence typically proceeds in stages. Each type of component is assembled as a group and then each group is soldered; and so on until all components are soldered. For example, the first step may be to assemble all surface mounted components on the top side and solder by infrared reflow. The next step may be to attach all components to the underside of the board with epoxy adhesive which requires heat to cause the epoxy to cure. The final step may require insertion of leaded components through the board and wave solder the inserted components and the surface mounted components that were previously attached with epoxy adhesive. In this typical sequence, the printed wiring board is subjected to three heating cycles, each of which causes degradation to the copper surface solderability.
Various methods have been proposed and used to preserve the solderability of the copper surfaces which are to be subjected to heat during an earlier cycle but are to be soldered in a later step during the process. Each of the prior art methods suffers from one or more disadvantages or problems.
One method is to coat the copper with a thin layer of solder, thus protecting the solderability. A problem with this method is that the thickness of the solder coating is critical; if the coating is too thick, it presents an obstacle to automatic placement of components on the surface. If it is too thin, non-solderable intermetallic coatings form on the surface.
A second prior art method is to coat the copper with a material such as benzotriozole and benzophenone or various types of imidizols. A disadvantage of this process is the tendency of these types of materials to lose their protective qualities during successive heating cycles.
A third method, known as the "preflux" method, is to apply a varnish-like material such as a rosin dissolved in a solvent, and drying to form a continuous coating on the copper which excludes moisture and, to a lesser extent, oxygen and their detrimental effect on solderability. Various methods have been used to enhance the stability of such coatings to prevent degradation during multiple heating cycles, among which have been development of synthetic rosins with known and predictable stability properties and the addition of anti-oxidants to prevent rosin breakdown. Such preflux formulations have included various combinations of rosins, solvents such as xylene, toluene, methanol, ethyl acetate, and various mineral spirit-based hydrocarbons, as well as antioxidants to stabilize the rosin. These prior art preflux formulations have problems in that the solvents have low flash points which present transportation and storage hazards. Furthermore, most of the solvents have been determined to cause health risks, and pollution when released to the atmosphere.
Hayes et al., U.S. Pat. No. 4,640,719, disclose methods and compositions for removing rosin soldering flux and adhesive tape residues from printed circuit and/or wiring boards and for testing the quality of curing of U.V.-cured soldermask on such boards. The compositions contain terpene compounds, preferably in combination with terpene emulsifying surfactants to facilitate removal by rinsing in water.
Futch et al., U.S. Pat. No. 4,934,391, disclose methods and composition for removal of solder flux, screen inks, and resists from contaminated surfaces, comprising dibasic acid esters such as adipic acid, succinic acid, and dimethyl glutarate, in combination with an appropriate emulsifying agent such as ethoxylated aliphatic alcohols, ethoxylated alkyl phenols, ethoxylated amines, and the like.